Lignin is the second most abundant organic compound on earth yet the use of lignin products is limited because of lack of manufacturing processes that readily isolate lignin in sufficient purity.
Sulfite pulping has been practiced since 1874. The early processes discharged dissolved wood sugars and lignin to rivers and streams causing significant pollution. Fermentation of sulfite liquor to hemicellulosic ethanol has been practiced, primarily to reduce the environmental impact of the discharges, from sulfite mills since 1909. The remaining spent liquor was concentrated to produce lignin rich slurry. This slurry was applied in gravel roads as a dust binder. Additional beneficial uses were developed to include surfactants, emulsion agent, dispersants, binder, etc.
The number of sulfite mills producing lignosulfonates was reduced because of new chemical recovery processes for sulfite spent liquors. Most of these processes require burning of the lignin. There are few remaining sulfite pulp mills in the world today that separate lignosulfonates, and the number of those remaining in operation continues to reduce each year. The research of beneficial uses of lignin byproduct has been reduced due to lack of available supplies.
Kraft chemical recovery pulping has eclipsed sulfite pulping as the dominant chemical pulping method. There are few commercial applications to precipitate lignin by acidifying pulping spent liquor using carbon dioxide. The recovery of Kraft pulping lignin by acid precipitation has been suggested, but Kraft lignin has undergone condensing reactions and does not possess the reactive properties of lignosulfonates.
Other processes have been suggested such as solvent pulping to produce pure lignin as a byproduct. The lignin extraction from solvent based pulping produces lignin that has not undergone significant chemical modification and is referred as “native lignin”. One such demonstration size facility for ethanol-water (ALCELL) pulping produced pure lignin in the early 1990's. The product was test marketed to be used in brake pads, etc. This plant suffered from uncontrolled precipitation of “sticky” lignin, which hardened if subjected to temperatures over 75° C. There are no plants currently in operation to produce pure lignin.
Therefore in the prior art of producing lignin are:                a) The sulfite processes where base calcium, sodium, ammonia or magnesium is retained with lignosulfonate;        b) Strong and dilute acid hydrolysis processing of lignocellulosic material, where lignin is the leftover solid after hydrolyzing cellulosic parts;        c) Kraft pulping process, where lignin is precipitated by acidifying spent pulping liquor;        d) Organic solvent pulping methods, where lignin is separated by removal of solvent.        
The present inventors have now developed a method wherein pure lignin can be readily produced from an organic solvent pulping or bioethanol process, This has been achieved through their experiments of organic solvent pulping with sulfur dioxide that produced lignin with unexpected properties. The lignin remained in colloidal suspension after removing the solvent and was hard to precipitate. Surprisingly, the lignin rapidly precipitated upon reheating the liquor at low pH, with and without further acidification. This precipitate filtered easily to form a whitish cake. Upon air drying the cake, the resulting lignin is a light colored powder, having a glass transient temperature at 160° C.-200° C. Furthermore, it was observed that the conditions were favorable to hydrolyze nearly all the hemicelluloses. This can be done in a batch process with a cycle time of between 0.5 and 3.5 hours, or in a continuous process.